Semiconductor device manufacturing processes includes a process that supplies a process gas into a reaction vessel to treat a substrate under a reduced pressure. An example such a process is a low pressure CVD (chemical vapor deposition) that deposits a thin film on a substrate through the reaction of film-forming gases. If a reaction product forming a thin film and reaction by-products are flown into an exhaust pipe and adhere to a gate valve (main valve) arranged in the exhaust pipe, leakage occurs in the valve when shutting off the valve. In order to avoid this, the exhaust pipe is provided with a trap on the upstream side of the valve to trap the above products. Adhesion of some specific sorts of reaction products and reaction by-products to the gate valve and the exhaust pipe can be prevented by heating them.
When a silicon oxide film (SiO2 film) is formed by using TEOS (tetraethyl orthosilicate: Si(O2C5H)4), decomposition products of non-reacted TEOS discharged from the reaction vessel adhere to the exhaust passage even if the exhaust passage is heated. In order to avoid this, a trap is arranged upstream of the gate valve.
In a CMOS (Complementary Metal Oxide Semiconductor), a silicon nitride (SixNy) film serving as a protective layer is deposited on a silicon oxide film serving as a gate insulating film. It has been examined, for forming the above two films, that the silicon oxide film is formed in a low pressure CVD system by using TEOS, and then the silicon nitride film is formed in the same low pressure CVD system by using dichlorosilane (SiH2Cl2) gas and ammonia gas.
The process pressure for the silicon nitride film formation is low, being lower than 133 Pa (1 Torr). Thus, a trap can not be provided in the exhaust passage, because the interior of the reaction vessel can not be evacuated to a target process pressure if the trap is provided. However, if a trap is not provided, decomposition products of non-reacted TEOS discharged form the reaction vessel during the silicon oxide film formation is likely to adhere to the valve in the exhaust passage. If the valve is a gate valve having pressure-controlling function, as rise in pressure in a space between the valve body and the valve seat unavoidably occur when the opening of the valve is small, decomposition products of non-reacted TEOS is likely to adhere to the surfaces of the valving element and the valve seat and then solidify. FIG. 6 shows a state in which solid matters originated from TEOS are deposited on surfaces around an O-ring 90 and a valve seat 92, on which a valving element 91 is seated, in a valve 9. If such solid matters are deposited on surfaces in the valve 9, the valving element 91 can not come in close contact with the valve seat 92 when the valve 9 is closed. Thus, leakage check of the reaction vessel to be carried out before the process becomes impossible. Therefore, frequent maintenance (notably, cleaning) of the valve 9 is necessary, and thus the operator bears a great burden.
JP11-195649A discloses that, immediately before closing a shutoff valve arranged in a source gas passage, a purge gas such as argon gas flows into the valve through a branch passage transversely connected the source gas passage, thereby to blow off solid matters adhered to surfaces facing a space between a valve seat and a valving element. Thus, leakage and damage of the valve seat, which may occur by closing the valve while solid matters are interposed between the valve seat and the valving element, can be prevented.
However, even if the purge gas is jetted to the valve in the source gas passage immediately before the valve is closed, only parts of the contact surfaces of the valve seat and the valving element are exposed to the purge gas having a velocity high enough to remove the solid matters. Moreover, if solid matters having high adhesion are adhered, a high removing effect can not be achieved.